I am trying to change my email on a site, because I can't log on my old email, and to go to the changing email screen I need to find out my birthday that I put down. At the time I just put a random birth date down to make a quick account but then I started using it more than I thought I would.

don't waste your time on bruteforcing this hash, it's 160 bits length or 10^19 combinations, it might take 584 942 years to find plaintext, so it's imposible to brute it, unless... you know the format which was used to generate that hash, or you could use wordlists.